Much attention has recently been directed to polyaniline, due in large part to its excellent stability and the relatively high levels of electrical conductivity of certain of its structural forms. For example, one form of polyaniline known as "emeraldine base" or "polyaniline base" (structure (1), Y=0.5) can be protonated (doped) by dilute aqueous protonic acid such as hydrochloric acid to produce the corresponding salt ( structure (2) , A=Cl). This salt exhibits conductivities of about 0.001-5 Siemans per centimeter (S/cm) as a compressed powder pellet. ##STR1## Polyaniline polymers can be processed into a variety of useful shaped articles such as fibers, films, and composites thereof. For many such applications, there is a positive correlation between increased molecular weight for the polyaniline employed and the performance properties of the resultant shaped article.
Currently, problems exist in the successful use of conductive polyanilines. For instance, the conductivity of the resultant polymers is still relatively low and increased conductivity levels would greatly advance the use of these materials in various industries. Also, to some extent, the ability to process high molecular weight polyaniline in the doped form presents problems for the useful application of these conductive polymers.
Melt processing of polyaniline has proven to be ineffective due to the decomposition of the polymer prior to its softening or melting point. Therefore, considerable research has been made in the area of solvents for the polyaniline. The emeraldine base form of polyaniline has been found to be soluble in such solvents as N-methylpyrrolidinone (NMP), certain amines, concentrated sulfuric acid, and other strong acids. However, it is preferred to process the polyaniline in its doped, conductive form to avoid the process of doping the resultant polyaniline form produced from the solution.
Recent advances in the solubility of the doped polyaniline salt are summarized in Counter-ion induced processibility of conducting polyaniline and of conducting polyblends of polyaniline in bulk polymers, Cao et al., Synthetic Metals, 48 (1992) 91-97. The authors report that polyaniline can be doped to its salt form and thereafter dissolved in common nonpolar or weakly polar organic solvents. This is accomplished by doping the base polyaniline in a functionalized protonic acid generally denoted as H.sup.+ (M.sup.- -R). The resultant doped polyaniline can then be dissolved in such listed solvents as xylene, CH.sub.3 Cl [sic, CHCl.sub.3 ], m-cresol, formic acid, and DMSO. The resulting films are reported to have relatively high conductivities. No explanation for the resulting high conductivities is suggested by the authors.
A need exists in the field to develop methods for preparing relatively high conducting polyanilines from either the base or salt form solutions in which the conductivity of the polyaniline can be greatly increased by predictable processing steps. Further, additional processes that can be followed for conversion of the base into a soluble polyaniline salt are also needed to increase processing alternatives.